The invention relates to a CCD image sensor arrangement having a semiconductor body comprising a charge-coupled device of a type in which there is transported between two successive signal charge packets which are representative of the local intensity of a trapped image a reference charge packet through the charge transport channel of the charge-coupled device which is representative of the overall smear charge and dark-current charge which is collected during the said charge transport.
CCD image sensor arrangements can be subdivided into a number of types. The best known types are the interline sensor and the frame transfer sensor. The interline sensor comprises a number of columns of photosensitive elements (pixels) in which the collected image can be converted during an integration period into charge packets, whose size corresponds to the local intensity of the collected image. Between the columns are arranged charge-coupled devices in which the charge-packets can be transported in vertical direction to a horizontal read-out register. Mostly, the charge-coupled devices are shielded against incident radiation by an opaque or a reflecting layer.
Image sensor arrangements of the frame transfer type mainly comprise only a large number of adjacent charge-coupled devices which are divided in the longitudinal direction into an image sensor section and a storage section. The image is projected onto the image sensor section and is converted into a pattern of charge packets, which after the integration period are rapidly transported to the storage section and are read out linewise by means of a horizontal read-out register.
Besides the types of image sensor arrangements mentioned here, there are other known types of CCD image sensor arrangements, such as, for example, image sensor arrangements of the line transfer type, to which apply the problems mentioned below and the solutions proposed in accordance with the invention.
A known problem which may arise in charge-coupled image sensors is the so-called smear, which can become manifest during the display by a vertical pattern of stripes in the image. This smear is due to the transport in the charge-coupled devices. During the transport, the charge packets pass the other image points in the same column and each time collect some charge, which is generated at or near these image points. This effect may even result in that, if locally the intensity is very high, a whole column becomes white during display.
A CCD image sensor arrangement of the kind described in the opening paragraph, in which the smear problem is solved for the most part with moving images, is known inter alia from Japanese Patent Application 208299/82 filed on Nov. 11, 1982, which has been laid open to public inspection on June 8, 1984 under Kokai No. 99882/84.
In this prepublished Application, a CCD sensor of the interline type is described, in which two charge-coupled devices are present between two adjacent columns of image points, the pixels in a column being alternately coupled to the two charge-coupled devices on either side of the column. Per charge-coupled device the number of stages, to be considered as a group of electrodes that can comprise one charge packet, is about twice the number of pixels which is associated with this charge-coupled device. As a result, it is possible to transport between the potential wells containing signal charge each time an "empty" potential well through the charge transport channel. The inverted commas for the word empty are provided to indicate that these potential wells do not contain signal charge and are "empty" only in that sense. During the charge transport, these "empty" potential wells pass the same locations as the potential wells which do contain signal charge and therefore collect the same smear charge as the signal charges. When now these smear charges are substracted from the charge packets stored in the potential wells containing signal charge, the signal charges can accurately be determined.
This known method can be used in interline sensors, but cannot be used in the sensors of the frame transfer type described above because in the latter type of sensors the charge-coupled devices constitute at the same time the photosensitive elements, in which the trapped radiation is converted during the integration period into electrical charge.
A further disadvantage of the known method is that the method of generating the smear charged proposed here requires each time an additional stage in the vertical charge-coupled devices, which occupies additional space on the semiconductor crystal and/or which renders it difficult to increase the number of pixels per unit length in the column direction and/or row direction.